There has been proposed an electrical connection structure with low-height, formed by fitting together a first connection member formed of a flexible printed circuit board and a second connection member (see e.g. JP4059522B; hereinafter referred to as “Document 1”).
The first connection member includes a thin insulation film having flexibility as a substrate, and pads formed on one surface (front surface) of the insulation film. Each of pads has elasticity and electrical conductivity. The insulation film is provided with through holes piercing the insulation film in the thickness direction. Each through hole is formed at a place of a corresponding pad. Each pad is provided with a pinhole communicating the corresponding through hole.
The second connection member has conductive protrusions each of which is shaped like a column. Each conductive protrusion is formed at a position facing a corresponding pad of the first connection member, and is adapted to be inserted into the pinhole of the corresponding pad.
When the first connection member is connected with the second connection member, columnar-shaped conductive protrusions are inserted into the pinholes of the respective pads. In this time, the pads are pressed by the conductive protrusions to be bent into the insertion direction of the conductive protrusions, and are made in elastic contact with the conductive protrusions by the restoring forces of the pads.
Recently, electronic devices have been desired to be more downsized, and electric components have been desired to be densely packed in a package. Therefore, there has been demanded an electrical connection structure having multiple electrical connections and having a compact size. In the electrical connection structure disclosed in Document 1, the plurality of pads are formed on one surface of the insulation film, and the plurality of conductive protrusions are formed on one surface (surface facing the plurality of pads) of the second connection member. In order to achieve an electrical connection structure having multiple electrical connections in a compact size, the plurality of pads (and the plurality of conductive protrusions) should be arranged in high density. In a configuration where the plurality of pads are arranged densely with a narrow pitch, a plurality of patterned conductors (circuit patterns) each extends toward outside from a corresponding pad cannot be tracked on only the one surface of the insulation film.
Considering this problem, it may be considered to form patterned conductors on the other surface (back surface) of the insulation film, and to connect each patterned conductor on the other surface with a corresponding pad on the one surface by means of a plated through hole. However, in this case, in order to electrically connect a patterned conductor on the front surface and a patterned conductor on the back surface with each other by means of a plated through hole, it needs to form a through hole in the insulation film and then to form plating on the inner surface of the through hole. In this process, a plated layer is formed not only on the back surface of the insulation film but also the front surface of the insulation film. It is therefore the thickness of each pad increases by the plate layer, and the pads become less likely to be bent. Therefore, there is a problem in this configuration that a relatively large stress is occurring in the pad when the pad is bent by the conductive protrusion.